Aluminum nitride (AlN) holds great promise as a semiconductor material for numerous applications, e.g., optoelectronic devices such as short-wavelength light-emitting diodes (LEDs) and lasers, dielectric layers in optical storage media, electronic substrates, and chip carriers where high thermal conductivity is essential, among many others. In principle, the properties of AlN may allow light emission at wavelengths down to around 200 nanometers (nm) to be achieved. Recent work has demonstrated that ultraviolet (UV) LEDs have superior performance when fabricated on low-defect AlN substrates prepared from bulk AlN single crystals. The use of AlN substrates is also expected to improve high-power radio-frequency (RF) devices made with nitride semiconductors due to the high thermal conductivity with low electrical conductivity. However, the commercial feasibility of AlN-based semiconductor devices is limited by the scarcity and high cost of large, low-defect single crystals of AlN.
To make large-diameter AlN substrates more readily available and cost-effective, and to make the devices built thereon commercially feasible, it is desirable to grow large-diameter (>25 mm) AlN bulk crystals at a high growth rate (>0.5 mm/hr) while preserving crystal quality. The most effective method of growing AlN bulk single crystals is the “sublimation-recondensation” method that involves sublimation of lower-quality (typically polycrystalline) AlN source material and recondensation of the resulting vapor to form the single-crystal AlN. U.S. Pat. No. 6,770,135 (the '135 patent), U.S. Pat. No. 7,638,346 (the '346 patent), U.S. Pat. No. 7,776,153 (the '153 patent), and U.S. Pat. No. 9,028,612 (the '612 patent), the entire disclosures of which are incorporated by reference herein, describe various aspects of sublimation-recondensation growth of AlN, both seeded and unseeded.
During seeded growth of AlN, single crystals of AlN are nucleated on a high-quality single-crystal seed, and the growing crystals replicate the crystalline order of the exposed face of the seed. Moreover, the diameter of the growing crystal is often allowed to expand beyond the diameter of the seed; such “diameter expansion” enables the growth of large crystals while obviating the need to utilize seed crystals with large diameters, which is beneficial since large-area AlN seeds are often prohibitively expensive or simply unavailable. Such diameter expansion may be enabled via control of the radial and/or axial thermal gradients within the growth chamber during crystal growth, as detailed in the '612 patent. Growth of single-crystal AlN boules is often initiated from AlN seeds having an exposed c-plane face with Al polarity, as the subsequent growth of AlN crystals with the Al-polarity orientation proceeds at a high growth rate—higher than growth rates enabled at other crystalline orientations. While such techniques produce boules of high-quality material, they are susceptible to parasitic, or “secondary” nucleation at the edges of the crystals. Parasitic nucleation events can result in higher defect densities, or even polycrystalline crystals, and thus the usable, high-quality area of the AlN crystal boule is often limited. Parasitic nucleation is a particular problem during diameter expansion of Al-polarity AlN crystals, as the radial thermal gradients that enable diameter expansion from small seeds may increase the frequency of parasitic nucleation events. Therefore, there is a need for improved crystal growth sequences that enable the formation of high-quality large single crystals of AlN.